Date: Thu, 23 Dec 2010 17:59:06 -0800
From: "Jim W7RY" <w7ry@inbox.com>
Subject: Re: [Amps] AL-1500 Plate Meter Circuit
The diode in this amp on the filter cap board shorted. That caused the grid
and plate current meters to read the same. Just the plate current divided on
the two meters.
Jim W7RY
## If that's the safety diode... wired between B- and chassis... Ameritron
sez in their
manual to check to see if it's shorted... as it's a wimpy 1N4007.
## On any of my HB amps..I use RVS connected 6A10's between B- and chassis.
Now if the one with the anode facing the chassis..shorts, because of a
glitch/surge.. or typ a B+ to chassis
arc/short,,, what happens is that shorted diode now provides a 'back door'
path for grid current to flow
through the shorted diode.... instead of the grid meter. It's easy to spot.
The amp puts out
normal power output.... but the grid meter reads zero ma. The plate meter
reads way on the high side,
and is actually reading combined plate + grid current [ = cathode current].
## with a XXX ohm resistor in series with the B+.... it's easy to calculate
the max fault current. I think the AL-1500
now uses a 25 ohm glitch..and with 3600v no load.... it's just 3600/25 =
144A of fault current..which will
short a 1N4007 in the blink of an eye. If it's only 10 ohm..then fault
current is 360A. [This assumes the new style
HV lytics are low esr types].
## I use a 50 ohm glitch in series with the B+. The safety diodes now
consist of 2-4 parallel 6A10's. 2-4 in parallel
6A10's with all their anodes facing the chassis. And 2-4 more in
parallel... with all their cathodes facing the chassis.
So 4-8 x safety diodes in total. That results in a 800-1600A surge
rating. OK, now you never have to replace the
damn things. They will never short. I also insert a fast, sand filled BUSS
HV fuse... in series, and just prior to the
50 ohm glitch R. Any B+ to chassis fault..or B+ to grid... and the 50 ohm
glitch LIMIT'S the fault current to a 'safe value'.
The HV fuse blows asap.. <2 msecs. The 6A10's safety diodes merely conduct
the fault current. 3500 v / 50 ohms = 70 A
of fault current. 70A will open up a fast 1A HV fuse...very quickly.
That amounts to a 7000% overload. Even with 6A10
safety diodes, rvs connected across the plate/grid meter [ or grid
shunt].... the fault path is always the chassis, up through the
ssafety diode... then back to B- of HV supply route... and nevr through the
grid-plate meter to B- route.
later... Jim VE7RF
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|